This invention relates to regenerative rotodynamic machines, and more especially to regenerative compressors and exhausters.
A regenerative or peripheral pump is a rotodynamic machine which permits a head equivalent to that of several centrifugal stages to be obtained from a single rotor with comparable tip speeds. The impeller can take the form of a disc with a set of flat vanes projecting axially at each side near the disc periphery. Around the greater portion of the periphery the vanes project into an annular channel of which the cross-sectional area is greater than that of the impeller vanes. Over a sector of limited extent between the inlet and discharge the annular channel is reduced to a close running clearance around the impeller. This sector is called the stripper seal and its function is to separate the inlet and discharge ports, thereby forcing fluid out through the discharge port. The stripper seal allows only the fluid between the impeller vanes to pass through to the inlet.
The advantage of pumps of this type lies in the generation of a high head at low flow rates. They have a very low specific speed. Although their efficiency is not very high, pumps of this type have found many applications in industry where it is preferred to use rotodynamic pumps in place of positive displacement pumps for duties requiring a high head at low flow rates. Their simplicity, the low noise levels generated and the absence of problems due to lubrication and wear, give advantages over positive displacement pumps, despite the lower efficiency.
The flat vane regenerative pump has been adapted for the compression of gas. The advantage lies in the low specific speed giving a high pressure ratio together with a low flow rate for a given size of machine. Further advantages are oil free operation and freedom from stall or surge instability.
In such a compressor, the gas follows a helical path through the annular channel and passes through the vanes a number of times in its peripheral path from the inlet port to the discharge port. Each passage through the vanes may be regarded as a stage of compression and thus the equivalent of several stages of compression can be obtained from a single impeller. However, this pumping process by means of flat vanes cannot be considered as efficient. The fluid between the vanes is thrown out and across the annular channel and violent mixing occurs, the angular momentum acquired by the fluid in its passage between the vanes being transferred to the fluid in the annular channel. The mixing process is accompanied by the production of a great deal of turbulence and this implies an undesirable waste of power.
Several early theories of the fluid-dynamic mechanism of a regenerative pump were published. These theories were reviewed and compared by Senoo (A.S.M.E. trans. Vol. 78, 1956, pp. 1901-1102). Differences occur in the assumptions made, but in principle the various theories appear to be compatible. Senoo and Inversen (A.S.M.E. Trans. Vol. 77, 1955, pp 19-28) considered turbulent friction between the moving impeller and the fluid as the primary force causing the pumping action. Wilson, Santalo and Oelrich (A.S.M.E. Trans. Vol. 77, 1955, 1303-1316) regarded the mechanism as based on a circulatory flow between the impeller and the fluid in the casing with an exchange of momentum between the fluid passing through the impeller and the fluid in the casing.
Much later, compressors with considerably better efficiency were proposed by Sixsmith ("The Theory and Design of a Regenerative Compressor" presented before The Institute of Refrigeration, May 1981) in which the previously conventional radial vanes are replaced by aerodynamic blading. The annular channel is provided with a core to assist in guiding the fluid so that it circulates through the blading with reduced loss. The core also acts as a shroud closely surrounding the blades at their tips to reduce losses due to the formation of vortices at the tips of the blades. An improved commercial compressor embodying inter alia these features is described in our EP No. 0011983 corresponding to U.S. Pat. Nos. 4,306,833 and 4,334,821.
Although the compressor of EP No. 0011983 has performed very satisfactorily, and better than its contemporaries in the field of commercial regenerative compressors, there remains room for further improvement. It is therefore a general object of the present invention to achieve a compressor of this class that has better performance while at the same time being easier and less expensive to manufacture.
Prior to the present invention, workers in the art of designing these aerodynamically-bladed machines were motivated by a number of preconceptions, thus:
(i) Noise in operation is always a problem and the Roots blower is an exceptionally noisy machine--therefore, design improvements to reduce noise will require a construction that is progressively less like a Roots blower;
(ii) The clearances between the moving parts are crucial to performance and must be maintained at a minimum in manufacture by expensive machining techniques.
(iii) The small clearances require special expensive precautions to be taken in choice of materials and manufacture in order to avoid the machine seizing with the considerable heat generated in operation.
(iv) High precision - fitting parts are also necessary to prevent radial leakages from the blade channel.